The present invention relates to methods for implementing the automatic slope and gain (ASG) control function in communications equipment, such as cable television distribution amplifier.
Those skilled in the art know that the attenuation of radio frequency (RF) signals in coaxial cable is, to an extent, a function of the temperature of the cable. Accordingly, it has long been the practice in cable television systems to provide gain correction at certain amplifiers in a cascade, which is a xe2x80x9cstringxe2x80x9d of amplifiers used to distribute signals to subscribers. Several methods of gain correction are used. The simplest gain correction method is known as thermal equalization. In this technique, the internal temperature of an amplifier station is sensed, for example by a thermistor, and the gain of the station is changed to approximate the change in cable loss that is assumed to result due to changes in ambient temperature, as represented by the change of temperature inside the amplifier. This technique is low in cost, but provides only for approximate compensation.
A more sophisticated method of controlling the amplifier gain involves sensing a signal level and adjusting the gain of the amplifier ahead of the sensing point to bring the level back to a standard, or expected, level. This is called an automatic gain control (AGC) system. The present invention is applicable to AGC systems, but present practice adds yet another level of sophistication called ASG. In ASG systems, it is recognized that the variation of attenuation with temperature exhibited by the coaxial cable is also a function of frequency. Accordingly, it is the practice to not only adjust the gain of the amplifier station according to the sensed signal level, but to adjust the gain vs. frequency characteristic (xe2x80x9cslopexe2x80x9d) of the station according to the sensed signal level.
Because the entire spectrum of signals on the cable is very wide (downstream transmissions in North America presently use frequencies from 54 to about 870 MHz.), it is common practice to sense only the level of one signal, called the xe2x80x9cpilotxe2x80x9d signal. Previous systems employed two pilot signals, one at the low end of the spectrum and one at the high end. Such systems were called dual pilot systems, but they are not common today due to reduced cascade lengths which resulted from employment of fiber optic transmission.
FIG. 1 is a block diagram of an amplifier station as they exist in the prior art. FIG. 1 shows an input path for the incoming signal to the amplifier input. This signal is received by input diplexer 1. Input diplexer 1 separates the signal into downstream (generally higher frequency) signals and reverse (usually lower frequency) signals. The reverse section of the amplifier usually does not employ ASG, but is merely shown for illustrative and complete diagram purposes, as will be appreciated by those skilled in the art. The remainder of this discussion will concentrate on the downstream path, shown at the top of the FIG. 1.
Downstream signals from input diplexer 1 are applied to input equalizer and attenuator 2, used for equalization of the signal. From the output of the input equalizer and attenuator 2, the signal is received by input amplifier 3. The input amplifier 3 is used to amplify the signal before downstream processing. The output of the input amplifier 3 is received by the interstage equalizer and attenuator 4. The interstage equalizer and attenuator 4 is used, as required, to shape the static response of the amplifier station and its output is received by ASG attenuator 5. The ASG attenuator 5 is used to apply the proper attenuation to the signal in accordance with its coordinating interstage amplifier 6 circuit. As explained above, the real circuit is usually more complex than just an attenuator. The real circuit usually includes components that shape the response such that there is more change in attenuation at high frequencies than at low frequencies. This is not shown here for clarity, but is well known by those skilled in the art.
Thus, from the attenuator 5, the signal is applied to a second amplifier stage, interstage amplifier 6 for further amplification in accordance with the needs of the pilot signal level. From interstage amplifier 6, the signal is sent to output splitter 8, which divides the signal into two different output paths. One path allows the signal to be applied, to a directional coupler 16, which takes a sample of the signal to the ASG detector 7. The ASG detector 7 detects the level of the pilot signal. The output of ASG detector 7 is a control signal which adjusts the attenuation of attenuator 5 such that the level of the pilot signal as measured by ASG detector 7 remains constant, regardless of the level of the incoming signal, within reasonable limits.
The pilot signal, having received correction, is directed by the directional coupler 16. The signal comes from directional coupler 16 and is applied to output amplifier A 9. The output of output amplifier A 9 is then received by output diplexer A 10. The diplexer having an input from the output amplifier A 9, and a reverse flow path to the reverse channel.
The path, yet to be discussed, from output splitter 8 goes through output amplifier B 11. From output amplifier B 11, the signal is received by output diplexer B 12. The output diplexer B 12, as does output diplexer A 10, separates the downstream and reverse signals as explained above.
Other amplifiers may have fewer or more outputs, and fewer or more amplification stages. Other variations on the architecture are well known to those skilled in the art. This block diagram is presented by way of example and not as a limitation to the applicability of the present invention.
FIG. 2 is a diagram showing an ASG detector known in the prior art. Note, signal flow in FIG. 2 is from left to right, while in the ASG detector 7 of FIG. 1, the signal flow is in the opposite direction. Signal input from directional coupler 16 (also see FIG. 1) is supplied to an input amplifier 21, used to amplify the input signal. The signal comes from the input amplifier 21 and is received by a bandpass filter 22, which selects the one signal that is to be used as the pilot from the plurality of carriers on the cable. As is well known in the art, the bandpass filter has a upper and lower limit of frequencies it will pass, forming an output referred to as passband of frequencies, or simply passband signal. After filtering, the signal may be amplified again, if necessary, by RF amplifier 23 the output of RF amplifier 23, is received by or detected by detector diode 24 and filter 25. The voltage on filter 25 is proportional to the level of the pilot carrier. Filter 25 also serves to pass the AC component from the detector circuit to a reference (ground). The voltage across filter 25, is compared with a reference voltage VRef 28 by loop amplifier 27. The feedback loop of this comparator circuit, the loop compensation 26, helps determine the dynamic response of the ASG loop, as is well understood by those skilled in the art. The output of loop amplifier 27 is supplied to ASG attenuator 5 (see FIG. 1) to control its attenuation. Thus, the ASG detector 7 and the attenuator 5 work to ensure that the level of the pilot signal remains constant at the output of the amplifier station of FIG. 1.
According to the teachings of the present state of the art, the ASG detectors utilize a circuit similar to that shown in FIG. 1. After the signal progresses through the input equalizer, amplifiers, and attenuators, the signal reaches the ASG detector. The prior state of the art teaches an ASG detector as shown in FIG. 2.
The system shown in FIG. 2 works, but has some limitations that are overcome by the present invention. The tuning of the ASG detector is set by the bandpass filter. This filter traditionally has been built with conventional inductors and capacitors (L-C filter) and recently there has been success at using surface acoustic wave (SAW) filters. However, with both of these technologies there are some limitations. With L-C filters, the first limitation is that it is difficult to design a filter that is narrow enough to preclude all except the one desired pilot carrier from reaching the detector. The typical frequencies used for pilot carriers today tend to range from about 450 MHz to about 650 MHz. In the architecture of FIG. 2, it is very difficult to design an L-C filter at these frequencies that is narrow enough to exclude signals located on adjacent channels, 6 MHz removed from the desired channel. This means that if the adjacent channel carriers are removed from the distribution spectrum for any reason, the operating point of the ASG system will change, possibly causing excessive distortion to be introduced into the plant. SAW filters can be realized with sufficiently narrow bandwidths such that this is not a problem.
A second limitation is that once a cable operator has chosen a pilot carrier frequency and has deployed equipment according to that selection, he is precluded from changing to a new pilot carrier. This has not been a major problem in the past, but does constrain an operator who may decide that another frequency would yield more satisfactory performance. A related and more serious problem, however, exists for the manufacturer of the equipment. Since different customers desire to use different pilot carrier frequencies, the manufacturer must design and build ASG detector circuits with different filters, tuned for different frequencies. When L-C filters are used, this is practical, although it creates continuing problems when a customer orders a frequency that is not in stock. When SAW filters are used, though, creating an ASG detector at a different frequency means paying non-recurring engineering charges to an outside firm to develop a new filter, then stocking different filters. It is common in manufacturing situations to get an order for a pilot frequency other than that provided by the SAW filters in stock and to have to order them. This can add weeks to delivery cycles, and increase costs for the manufacturer, as well as inconveniencing the cable operator customer.
Yet another limitation of the present art is that some day the cable industry likely will change the signal on the pilot frequency from analog modulation to digital modulation. Today, it is the traditional practice of cable systems to carry television signals in analog format, with a picture carrier located 1.25 MHz above the lower channel boundary, a color carrier located 3.58 MHz (for the NTSC television system used in North America) higher than the picture carrier, and a sound carrier located 4.5 MHz above the picture carrier. Today all pilot carriers we are aware of are utilizing this kind of analog modulation. The optimum design of the ASG detector is to center the bandpass filter on the picture carrier of the pilot channel.
However, the industry has begun transmitting some signals using digital modulation, which occupies the channel in a much different manner. The tuning of a bandpass filter for analog signals is no longer optimum if the analog-modulated pilot signal is replaced by a digital-modulated pilot signal. Furthermore, it is current practice to set the level of the digital signal significantly lower than the level of the analog-modulated signal, particularly as read by the detector. This means that if the analog pilot signal is replaced with a digital signal, the ASG operating point would shift drastically, thereby causing incorrect operation of the amplifier. Therefore, it is and will be increasingly necessary to have a reception method that corresponds to the signals transmitted. The reception method being 1) setting the reference level, and 2) tuning to a specific frequency.
At the present time, the cable television industry is not immediately planning to replace analog signals at the pilot frequency with digital signals, but it is inevitable that this will occur in the future. It is desirable to take action when upgrading the plant now, which will ease the operational burden of modifying the ASG circuits for digital modulation when analog signal at the pilot frequency are replaced with digital signals.
FIG. 3 shows the difference between analog-modulated and digital-modulated carriers. FIG. 3 is a conventional spectrum plot of two TV channels. The lower TV channel 55 is modulated with an analog signal, and the higher channel 56 is modulated with a digital signal. The analog channel is characterized by a picture carrier 51, a color carrier 58, and a sound carrier 59. The preferred ASG detector passband 52 for the analog signals is centered on the picture carrier as shown. Using a SAW filter 22 (FIG. 2.), this desirable passband shape can be approximated rather well. When using an L-C filter, the passband is wider.
Shown to the right of the analog-modulated channel in FIG. 3 is TV channel 56, which is carrying a digital-modulated carrier. The spectrum of a digital channel 57 is much different. The spectrum 57 is occupied over most of the channel by a signal that has a constant power for every unit of bandwidth. The passband 53 of the ASG system is near the center of the channel, because as one approaches the channel edge, the energy density is no longer constant. The digital carrier 54 (usually suppressed) is shown approximately centered in the spectrum 57. The difference in the modulation methods may be readily observed. Thus, when the respective pilot signals are changed, the need for the present invention becomes apparent to one skilled in the art.
Furthermore, the signal level of the digital signal is intentionally set lower than that of the analog signal to reduce the loading on the distribution plant. Typically, 64 QAM modulated signals are set about 10 dB lower than the analog signals, and 256 QAM signals might be set 6 dB lower. These numbers could change in the future.
Because the desirable passband for the ASG detector is narrow compared with the occupied bandwidth, a detector will have to detect even lower amounts of energy. Those skilled in the art know that the majority of the energy in an analog signal is concentrated in the picture carrier. As a practical matter, if one uses a filter having a passband 300 KHz wide, one will successfully detect the peak amplitude of the analog signal. However, if one uses this filter for a digital signal having a total bandwidth of 5 MHz, those skilled in the art know that the power reaching the detector will be further reduced by       10    ⁢          xe2x80x83        ⁢    log    ⁢          5000      300        =      12.2    ⁢          xe2x80x83        ⁢          dB      .      
Thus, the digital signal is intentionally set 6-10 dB lower than the analog signal it replaces, and the ideal detector will experience a further reduction of the detected signal level by 12.2 dB. Clearly, where it is desired to keep the output within xc2x11 dB of the desired level, some sort of accommodation must be made if the pilot carrier modulation is changed from analog to digital.
Furthermore, suppose that the pilot is converted to a digital signal, but the adjacent channel continues to carry an analog signal, which is a likely scenario during conversion from analog to digital. The higher analog signal on the adjacent channel would require the performance of a filter to be even better. It would be desirable if, at the time of conversion of the pilot channel from analog to digital, a signal could be sent from the headend that would automatically retune the detector from the analog picture carrier to the approximate center of the channel, and would also change the ASG set-point, determined by a reference voltage, to that appropriate for the digital signal. The reverse could be accomplished in a like manner, were the less likely change made from a digital-modulated channel to an analog modulated channel.
Finally, while present ASG circuits are reasonably effective, they have limitations in how accurately they can hold the output signal level as a result of temperature changes in their own circuitry. Input amplifiers, RF amplifiers, filters, and detectors all exhibit some variations in gain with temperature. These variations represent errors in the output signal level that cannot be corrected by the ASG system. While it is possible to include temperature compensation in the system, the practical amount of correction is somewhat limited.
The present invention overcomes the above described problems by utilizing a circuit, and a method to allow flexibility in the changing carrier frequencies that may be used in a coaxial cable feed for ASG sensing;, with minimum disruption during the changing of that carrier frequency.
The present invention includes using an agile tuner to select the frequency that is used for ASG sensing (the xe2x80x9cpilot signalxe2x80x9d), thereby allowing a cable operator to change the pilot carrier frequency from time to time. This is useful if the reference pilot carrier frequency is changed or if the chosen pilot carrier is changed from analog to digital at some point in the future. It also provides manufacturing efficiency in that the manufacturer is not required to stock multiple filters for different pilot frequencies. A second aspect of the present invention includes shifting the signal detector set-point such that if the pilot carrier is changed from having an analog carrier to having digital modulation, the amplifier""s operating point can be changed without having to physically visit the site. A further aspect of the present invention includes using a micro-controller (low end microprocessor) to control the set-point of the amplitude detection circuit. This aspect can then be expanded to provide useful improved thermal compensation and low cost dual slope AGC, such as by using a microcontroller to improve the performance of a thermal equalizer, which is used where an ASG circuit is not used. The microcontroller can be used to improve temperature compensation of the ASG circuit itself.